1. Field of the Invention
The present invention relates to metal nanowires with high linearity and a transparent conductive film including the same. More specifically, the present invention relates to metal nanowires with high linearity that have excellent electrical properties and high transmittance; and a transparent conductive film that includes the metal nanowires and is thus suitable for use as a transparent electrode for a variety of electronic devices, including flexible electronics, and energy devices.
2. Description of the Related Art
Transparent conducting materials (TCMs) should exhibit a high transmittance (≧90%) in the visible wavelength range of 380 to 780 nm and a low resistivity (˜2×10−4 Ω/cm). These characteristics enable the wide use of transparent conducting materials in transparent electrodes for displays, digital TVs, next-generation mobile communication devices, solar cells, touch panels, OLEDs, and the like.
Metal oxides, such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO), are typically used as representative transparent conducting materials. Thin films of metals, such as Au, Ag, Cu and Ni, are currently under study as transparent conducting materials.
ITO, which is presently the most widely used type of transparent conducting material, has high electrical conductivity, good chemical stability and excellent electrical and optical properties, such as high transmittance. However, since indium as a principal material of ITO is an expensive limited resource, a new transparent conducting material as a substitute for ITO is strongly needed.
Particularly, most ITO thin films are formed by physical vapor deposition (PVD) techniques, such as sputtering and E-beam evaporation, which require high temperatures of 400° C. or more or heat treatment. This heating leads to high production costs of ITO thin films and makes it difficult to form ITO thin films on highly flexible plastic substrates and films. Further, ITO films deposited on polymer films are not satisfactory in terms of flexibility and are readily degraded when exposed to plasma, resulting in deterioration of their characteristics.
Research is being undertaken on the use of conductive polymers as organic materials for transparent electrodes. However, the electrical conductivity of conductive polymers generally increases in proportion to the thickness of electrodes. Conductive polymers absorbing light in the visible region should be coated as thinly as possible to achieve high transmittance sufficient for use in displays. However, a sheet resistance of 1 kΩ/sq. or less is generally not obtained in a 50 nm thick coating of a conductive polymer on a substrate. A thicker coating of the conductive polymer is required to obtain a lower sheet resistance. That is, a conductive polymer having a high transmittance in the visible region does not meet electrical properties required in application fields of transparent electrodes.
In an effort to solve such problems, the present inventor has tried to develop a technique for producing nano-scale wires of metals, such as Au, Ag, Cu and Ni, that have improved conductivity while minimizing the scattering of light in the visible region.
As the prior art, U.S. Patent Publication No. 2008/0210052 A1 discloses a method for producing nanowires of metals, such as Ag, Au and Pt, having a diameter of 75±8 nm and a length of 9.7±2.6 μm from a reaction mixture including a metal salt, a capping agent and an ammonium compound in a reducing solvent. Further, U.S. Patent Publication No. 2008/0286447 A1 discloses a method for forming a transparent conductive film using metal nanowires.
Hydrothermal synthesis, chemical vapor synthesis and polyol methods have been widely investigated to date as methods for synthesizing metal nanowires. Chemical vapor synthesis necessitates the use of expensive equipment for mass production and requires an additional process for the removal of by-products after synthesis. In contrast, chemical vapor synthesis and polyol methods are simple to carry out and enable mass production of metal nanowires at reduced costs. Despite these advantages, hydrothermal synthesis has a problem in that aggregation of particles in aqueous solutions tends to increase. Other problems of hydrothermal synthesis are that particles are not uniform in size even after drying and are not readily dispersible, making it difficult to prepare coating solutions.